1. Field of the Invention
This invention relates to a differential pressure indicator for indicating when the difference in fluid pressure between two locations exceeds a predetermined level. More particularly, it relates to a differential pressure indicator which does not operate when the fluid viscosity is high.
2. Description of the Related Art
In fluid systems equipped with a filter element, it is frequently important to determine when the filter element has become clogged by particles in the fluid being filtered. For example, clogging of an oil filter for a jet engine is undesirable because it can result in significant wear or even serious damage to the moving parts of the engine. Clogging of a filter element can be detected by measuring the pressure drop (the pressure differential) across the filter element, which increases as the clogging progresses. Therefore, it is common to monitor the condition of the filter element by measuring the pressure differential between the upstream and downstream sides of the filter element with a differential pressure indicator. When the pressure differential reaches a predetermined value indicative of clogging of the filter element, the filter element can be replaced or cleaned. Differential pressure indicators for this purpose have been developed which actuate at a predetermined differential pressure across a filter element to produce a visual or electrical signal which alerts a user of the filter system of the need to replace the filter element.
The differential pressure across a filter element is a function not only of the degree of clogging of the filter element but also of the viscosity of the fluid being filtered. Namely, the higher the viscosity, the greater is the differential pressure. With many fluids, viscosity increases as the fluid temperature decreases. For example, at the time of start up of an engine, the lubricating oil for the engine is much colder and more viscous than at normal engine operating temperatures. During start up, the high viscosity of the oil may cause the differential pressure across an oil filter to exceed a value indicative of clogging of the filter element, even though the filter element is not actually clogged. Accordingly, in order to prevent a false indication of clogging, a differential pressure indicator for a filter system is frequently equipped with an arrangement referred to as a thermal lockout device which prevents actuation of the indicator when the fluid is at a temperature producing a high viscosity.
A typical thermal lockout device for a differential pressure indicator employs a bimetallic element to prevent actuation of the indicator when the fluid of which the pressure is being measured is below a prescribed temperature. However, a bimetallic element is difficult to adjust. Furthermore, the bimetallic element is usually in contact with the atmosphere rather than directly in contact with the fluid being filtered. Therefore, the bimetallic element can not accurately sense the temperature of the fluid.